Biology ReviewCellular Work and Related Processes

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Much of the text material is from, “Essential Biology with Physiology” by Neil A. Campbell, Jane B. Reece, and Eric J. Simon

(2004 and 2008). I don’t claim authorship. Other sources were also used and are noted.

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Outline

• Harvesting of chemical energy• Enzymes and enzyme inhibitors• Cell membrane transport

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Harvesting of Chemical Energy

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Chemical Energy

• Food, gasoline, and other fuels are forms of chemical energy, a type of potential energy.

• Carbohydrates and fats have carbon backbones that make them rich in chemical energy.

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Harvesting of Chemical Energy

• Cells and internal combustion engines use similar physical processes to perform work.

• A gasoline engine mixes oxygen with octane in an explosive chemical reaction that breaks-down the covalent bonds in the carbon backbone for rapid liberation of energy.

• The reaction moves the pistons in the cylinders, which ultimately drives the wheels.

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Harvesting of Chemical Energy (continued)

• Cells use oxygen to harvest chemical energy from food molecules in a more controlled process known as cellular respiration.

• Aerobic cellular respiration, which requires oxygen, occurs in the mito-chondria of eukaryotic cells.

• Anaerobic respiration, which does not use oxygen, occurs in the cytosol of prokaryotic and eukaryotic cells.

• In both types of respiration, the molecule ATP (adenosine triphosphate) is generated as a chemical energy source for cellular work.

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ATP Molecule

http://biology.clc.uc.edu

Adenosine triphosphate (ATP)

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Efficiencies

• Automobile engines extract about 25 percent of the chemical energy to produce kinetic energy to drive the wheels—the rest is converted to heat.

• Cells extract about 40 percent of the chemical energy from food mole-cules to perform cellular work.

• The waste, or byproducts, of internal combustion engines and cellular respiration are mostly CO2 and water.

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Cellular Work

• Cellular work consists of maintaining the body’s metabolism (sum of all its chemical processes) and powering muscle contractions.

• The remaining 60 percent of the chemical energy generates body heat to maintain the body at a constant temperature—about 98.6oF or 37oC.

• Sweating and other cooling mechanisms enable the body to lose excess heat including in hot environments and physical exercise.

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Life Requires Chemical Energy

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Calories

• Calorie is a unit of chemical energy used in the physical and biological sciences.

• A calorie (c) is the amount of energy required to raise the temperature of one gram of water by one degree Celsius (oC).

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Calories (continued)

• The caloric content of food is measured by burning it completely to ash under a container of water, and measuring the increase in the water temperature.

• A handful of peanuts has enough chemical energy to boil more than a quart of water if the peanuts could be completely converted to heat.

• A calorimeter is used by food scientists to measure the caloric con-tent of foods.

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Bomb Calorimeter

http://chemistry.umeche.maine.edu

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Kilocalories

• The use of calories (c) to express the energy content of food is not practical since a calorie is a very small unit of measurement.

• The daily recommended diet for adults would be about 2.0 x 106, or two million calories.

• Instead, kilocalories (kcal or C) are used in which one kilocalorie is equals to 1,000 calories (c).

Kilo- = 1,000

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Kilocalories (continued)

• The daily recommended diet for adults is about 2,000 kilocalories (C).

• Gender, age, basal metabolic rate, physical activity, and other factors determine recommended caloric intake.

• The calories listed on food labels are always expressed in kilocalories.

• A way to minimize confusion between c and C is to refer to kilocalories as food calories.

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Food Label

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Caloric Densities

http://www.asymptotia.com

http://www.wisegeek.com

Each plate contains about 200 food calories.

Although certain foods may have about equal food caloric content,

they can differ substantially in caloric densities.

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Caloric Accounting

• Caloric accounting considers a person’s food intake, basal metabolic rate, and physical activity.

• Caloric imbalances between food intake, and basal metabolic rate and physical activity can lead to weight gain or weight loss.

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Calorie Expenditures

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600 kcal (fast), 200 kcal (slow)

535 kcal (2 mph)

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160 kcal (3 mph)

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510 kcal (fast) �170 kcal (slow)

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Enzymes and Enzyme Inhibitors

http://www.unc.edu

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Metabolism and Enzymes

• The sum of all chemical reactions in an organism is known as its metab-olism.

• Enzymes are specialized proteins that lower activation thresholds and speed-up many types of chemical reactions.

• The covalent bonds in molecules must be broken to initiate a chemical reaction.

• Covalent bond breakage occurs, for example, when a disaccharide (a double sugar) is hydrolyzed into two monosaccharides (single sugars).

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Enzymes

• Energy, usually in the form of heat, is needed for a chemical reaction to occur—the threshold amount of heat is called its activation energy.

• Adding substantial amounts of heat is often not possible or desirable with living cells.

• Enzymes enable metabolism to occur at lower temperatures by reduc-ing the amount of activation energy required to break molecular bonds.

• Enzymes are catalysts that lower the barriers for chemical reactions to occur.

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Induced Fit

• An enzyme is specific in the chemical reaction it catalyzes although thousands of different chemical reactions occur in the human body.

• The active site of an enzyme has a shape that fits a portion of the substrate molecule, much like the correct key readily opens a door lock.

• As an enzyme attaches to the substrate, it changes its shape slightly to enable a physical embrace between the molecules in what is called induced fit.

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Breaking of Covalent Bonds

• The enzyme places the substrate under physical or chemical stress, making it easier to break the covalent bonds and initiate the chemical reaction.

• Once the covalent bonds are broken, the enzyme molecule can bind with another substrate molecule to begin the process again.

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Enzyme Inhibitors

• Certain types of molecules can inhibit metabolic reactions by binding to enzymes and disrupting their functions.

• Enzyme inhibitors are specific to the enzymes they target.

• Some inhibitors are imposters of substrates that bind to enzymes.

• Other inhibitors bind to a different part of the enzyme and change the shape of the active site so that it can no longer bind to the substrate.

• Organisms produce enzyme inhibitors to control the overproduction of enzymes.

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Enzyme Inhibitors (continued)

• Enzymes inhibitors are manufactured for many medical and biological purposes.

• Malathion, an insecticide, inhibits an enzyme for the functioning of insect nervous systems.

• Aerial spraying of malathion has been used to control Mediterranean fruit fly infestations in southern California—it turned-out to be a controversial program.

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American Civil War

http://www.a2zcds.com

http://nmhm.washingtondc.museum

During the American Civil War, many soldiers died from bacterial infections in the treatment of their wounds—possibly as many as

died in battle.

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Antibiotics

• Antibiotics are derived from microorganisms that disable or kill bacteria.• In the 1920s, Alexander Fleming discovered penicillin when he found a

mold prevented the growth of bacteria that he was trying to cultivate in bread.

• Penicillin, the first antibiotic to be developed, inhibits an enzyme needed to form the cell walls in bacteria.

• The death rates from diseases such as bacterial pneumonia and surgi-cal infections dropped substantially once antibiotics were widely avail-able. �

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Targets of Other Antibiotics

• Ampicillin and bacitracin—bacterial cell walls.

• Erythromycin, streptomycin, and tetracycline—bacterial ribosomes.

• Ciprofloxacin—bacterial chromosomal structure.

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Cell Membrane Transport

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Cell Membrane Transport

• Cells can control the flow of materials across their plasma membranes.

• A major function of the plasma membrane, in addition to providing the cell boundary, is regulating the movement of molecules into and out of the cell.

• Three forms of transport are: diffusion, osmosis, and active transport— the first two are passive processes that do not require chemical energy.

http://upload.wikimedia.org

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Diffusion

• The heat energy of molecules causes them vibrate and this move ran-domly in what is called Brownian motion.

• A result of the motion is diffusion, the tendency of molecules to spread into the available space.

• Although each molecule moves randomly, the overall movement is in one direction, from high- to low-concentration, along its concentration gradient.

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Diffusion and Directional Movement

• The directional movement can be shown with movement of dye across a semi-permeable membrane in a container of water

• The membrane has pores large enough to pass the dye molecules but not the water.

• An equilibrium exists once the dye is evenly diffused—the number of molecules moving across the membrane is now about the same in both directions.

• Two different dyes will diffuse along their own concentration gradients as if the other molecule did not exist.

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Concentration Gradients

Passage of time

Concentration gradient

Individual concentration gradientsRed

Green

Passage of time

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Passive Transport

• Diffusion across a semi-permeable membrane is a form of passive transport since it does not require expenditure of chemical energy.

• A cell’s plasma membrane is selectively permeable to some mole-cules.

• The membrane allows certain small ions to pass (such as Na+ and K+), but not large molecules such as proteins and phosphate groups.

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Passive Transport (continued)

• Passive transport is an important process for maintaining all living cells.

• For example, O2 enters the hemoglobin of red blood cells through passive diffusion to be transported in blood to meet the metabolic needs of the body’s tissues.

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Osmosis

• The passive transport of water across a semi-permeable membrane is called osmosis.

• Consider a membrane separating two compartments that is permeable to H2O but not to C6H12O6 (glucose), a larger molecule.

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Osmosis (continued)

• The solution with a higher concentration of solute (in this case, glucose) is hypertonic and the solution with a lower solute concentration is hypo-tonic.

• H2O will diffuse across the membrane from the hypotonic solution to the hypertonic solution until equilibrium is established.

• The solutions are isotonic once they reach equal concentrations of glu-cose.

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Osmosis (continued)

H2O molecules are small enough to pass through the semi-permeable membrane, but the glucose molecules cannot pass

because they are much larger.

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Water Regulation

• The survival of cells depends on the body’s ability to regulate water uptake and loss.

• When red blood cells are immersed in an isotonic solution, the volume of the cells remains constant.

• Hypotonic and hypertonic environments can cause a cell to expand and burst or shrivel and die.

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Red Blood Cells

http://www.ccs.k12.in.us

Distilled water = hypotonic solution.Salt water = hypertonic solution.

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Active Transport

• Active transport requires the expenditure of chemical energy to move molecules across a cell’s plasma membrane.

• A transport protein in the plasma membrane, using ATP as its energy source, pumps the solute across the membrane against its concentra-tion gradient.

• Active transport enables cells to maintain intracellular concentrations of molecules that differ from the extracellular environment.

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Active Transport (continued)

• A cell generally has a higher concentration of potassium (K+) ions and lower concentration of sodium (Na+) ions in the intracellular space.

• The concentration differences are regulated by the sodium-potassium pump of transport proteins.

• This pump is vital in enabling neurons to generate nerve impulses—or action potentials—as we will discuss in an upcoming lecture.

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Exocytosis

• Large molecules, including many proteins, are too large to fit through the plasma membrane.

• Transport vesicles carry proteins manufactured by the ribosomes, and fuse with the plasma membrane to empty their contents outside of the cell.

• The process is known as exocytosis.

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Endocytosis

• The opposite process is endocytosis—cells take in materials such as food molecules and water in vesicles that bud inward from the plasma membrane.

• Endocytosis and exocytosis both require chemical energy, and thus are active processes.

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Receptor-Mediated Endocytosis

• Another type of endocytosis is when certain external molecules bind with receptor proteins in the plasma membrane to be transported into the cell.

• This process is called receptor-mediated endocytosis.

• In a genetic disorder, plasma membranes of cells cannot take-up sufficient amounts of cholesterol bound to low density lipoproteins (LDL).

• High LDL levels result, which can lead to cardiovascular problems if left untreated or inadequately treated.

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Cell Signaling

• Many types of cells can communicate with each other across their plasma membranes.

• A signal from outside of the cell—such as a water-soluble hormone—is received by receptor proteins in the plasma membrane or cytoplasm.

• The signal triggers a chemical chain reaction inside the cell, in what is known as a signal transduction pathway.

• The signal can lead to responses such as metabolic changes in the cell or rearrangement of the cytoskeleton.

• Cell signaling is a key mechanism for many hormones of the endocrine system.